Photosynthesis

Question 1

Light-dependent reaction

Answer 1

1. Light is captured by photosynthetic pigment molecules in light harvesting complex of chloroplast

2a. Excitation of electron to higher energy level and
b. electron subsequently passing along a series of electron carriers (ETC) and to final electron acceptor NADP+
=> NADP+ is reduced to NADPH

3. Energy released from electron transport is used to pump H+ protons
   across the thylakoid membrane
   from the stroma into the thylakoid lumen

4a. Proton gradient is generated and
4b. Since there is a tendency for H+ to diffuse back into stroma, proton motive force generated

5a. protons re-enter stroma via ATP synthase complex
(as thylakoid membrane is impermeable to H+ since H+ is hydrophilic and unable to pass through hydrophobic core)
b. ATP synthase uses energy from the proton gradient to synthesise ATP from ADP and Pi

Question 2

Basic details of light-independent reaction

Answer 2

Involves Calvin Cycle, which occurs in stroma to reduce CO2 using ATP and NADPH

Question 3

Phase 1 of Calvin Cycle

Answer 3

Carbon fixation
a. 1 molecule of CO2 combines with ribulose biphosphate (RuBP) to form an unstable 6C intermediate
b. which breaks down into 2 molecules of glycerate-3-phosphate (PGA)

Question 4

Phase 2 of Calvin Cycle

Answer 4

Reduction of GP
a. Each molecule of GP is phosphorylated by ATP to form 1,3-biphosphoglycerate (BPG)
b. which is then reduced by NADPH to form glyceraldehyde-3-phosphate (G3P/GP)

Question 5

Phase 3 of Calvin Cycle

Answer 5

Regeneration of RuBP
a. Every 3 molecules of CO2 will combine with 3 RuBP to eventually form 6 G3P
b. Only 1 G3P counted as gain of carbohydrate, as other 5 will be used to regenerate 3 RuBP
=> Calvin Cycle continues

Question 6

Elaboration of electron flow (in photosynthesis)

Answer 6

Details:
1. PS (I and II) and ETC are embedded in thylakoid membrane
2. PS (I and II) consist of chlorophyll a and accessory pigments used to harvest light energy
3a. ETC consists of membrane proteins functioning as electron carriers
3b. ETC is arranged in increasing electron affinity
3c. thus ensuring flow of electrons down ETC is unidirectional

Process:
1. light energy excites an electron in PS (I or II) to higher energy and captured by primary electron acceptor
2. electrons are then passed down electron carriers through a series of redox reactions
3. until it reaches the final electron acceptor NADP, reducing it to NADPH

Question 7

How quality of light affects photosynthesis

Answer 7

1) light wavelength
- presence of different pigments in plants to effectively increase range of wavelengths from which plants can obtain energy to drive photosynthesis
- since each pigment can only absorb certain wavelengths of light
- e.g. blue and red light are wavelengths optimally absorbed by chlorophyll
- e.g. intermediate light are wavelengths absorbed by carotenoids (accessory pigments)

2) light intensity
- as light intensity increases, more light absorbed by photosynthetic pigments

Question 8

what happens to products of photosynthesis

Answer 8

• Carbohydrates:
  
  • 2 G3P used to synthesis 1 glucose molecule
  • which will be polymerised into starch and cellulose
  • for building of new tissues/organs resulting in plant growth
• Others:
  
  • G3P converted to lipids/amino acids/proteins
  • amino acids used to make proteins for growth/cell division
  • lipid, as well as glucose, used as respiratory substrate to release ATP for cell division/growth

Question 9

why does [RuBP] increase when [CO2] is limiting

Answer 9

• lower [CO2]
• less CO2 combine with RuBP
• while RuBP continues to be reformed from GP